According to http://greenbatteries.com/nibafa.html
Does rapid charging reduce the life of batteries?
No. So long as it is done using a properly designed, smart charger, most NiMH batteries can be recharged in about an hour without any damage or reduction in their life.
I have a 2-hour charger from walmart rated at 700 mah. I use it charge batteries from 1600-2500 mah. After the batter is charged the charger automatically shuts off. Is it worth it for me to spend $40-$50 on a more expensive slower charger?